Allow $meta args in sort() so text search sorting works

Hey Aaron,

Ran into this issue while trying to do text search with Mongoose. Sorting by text search score as described in http://docs.mongodb.org/master/reference/operator/query/text/#op._S_text doesn't work right now in mquery, or by extension Mongoose, against MongoDB 2.6.0-rc2. Would be nice for this to work for the MongoDB 2.6.0 release.